1. Field of the Invention
This invention relates to a sliding sheet material for high-temperature use which is to be interposed between members in sliding contact with each other, such as a bearing bushing, a washer and a packing.
2. Related Art
For example, an exhaust conduit of an automobile engine is heated to high temperatures, and also vibrations of the engine are propagated to the exhaust conduit. Therefore, it is a common practice in the art to interpose a sliding member, such as a packing, at an interconnecting portion of the exhaust conduit where exhaust pipes are serially connected together, so as to prevent a gas leakage and also to dampen the vibrations.
JP-A-60-84477 (prior art technique 1) discloses a sliding member such as a packing, in which a band-like, tubular matrix and a band-like, sliding face-forming member are placed one upon the other (that is, stacked together), and are compressed and deformed, so that metal wires are partially exposed to the surface of the sliding member. The tubular matrix is formed by superposing a wire net of a thin stainless steel wire on a heat-resistant sheet material made, for example, of exfoliated graphite, and then by coiling these materials into a roll-like configuration, and then by crushing and flattening the materials of the roll-like configuration into a band-like configuration. The sliding face-forming member is formed by the steps of placing a heat-resistant sheet, made for example of asbestos, on an exfoliated graphite sheet, applying a powdered solid lubricant onto this heat-resistant sheet, overlaying a composite net (formed by weaving or knitting a thin metal wire and a fluororesin thread together) on these materials, and crushing and flattening the stacked materials into a band-like configuration. Then, the two members thus produced are stacked together, and are compressed and shaped into the packing. Therefore, this sliding member has the sliding surface on which the fluororesin, the thin metal wire and the solid lubricant are present in a mixed manner, and therefore this sliding member exhibits excellent sliding properties over a long period of time.
A packing, disclosed in JP-A-1-307591 (prior art technique 2), is formed by superposing one wire net (expanded metal) on one sheet of exfoliated graphite, and then by coiling these materials into a roll-like configuration, and then by compressing and shaping the coiled materials into a final shape of the packing. Therefore, the packing can be easily produced, and is less susceptible to damage.
In both of the sheet materials (packings) of prior art techniques 1 and 2, the wire net and the exfoliated graphite sheet are stacked together, and then are coiled into a roll-shaped configuration. Therefore, each of the resultant sheet materials has an increased thickness, and there has been a limit to the formation of these sheet materials into a reduced thickness. And besides, the construction of these sheet materials is complicated, and their cost is high.
There is already known a sliding sheet material of the type in which one wire net is superposed on one exfoliated graphite sheet, and then these are compressed, so that the wire net is covered with the exfoliated graphite sheet, with the exfoliate graphite sheet filled in the meshes (interstices) in the wire net. More specifically, the filling with respect to this sliding sheet is effected in the following manner. The wire net 51 is superposed on the exfoliated graphite sheet 50 as shown in FIG. 7A, and then the compression is effected as shown in FIG. 7B. At this time, if the compression is excessive, not only an obverse side 50a of the exfoliated graphite sheet 50 but also a reverse side 50b thereof are cut by metal portions forming the meshes of the wire net 51, as shown in FIG. 7C, so that the exfoliated graphite sheet 50 is liable to become separated from the wire net 51. Therefore, at the time of effecting the compression, the compressive force is so adjusted that the reverse side 50b will not be cut, as shown in FIG. 7D. Namely, only the obverse side 50a is cut, and the reverse side 50b is kept continuous by a layer portion "n" which is not cut.
Therefore, the wire net is not exposed to the reverse side 50b, and is covered with the exfoliated graphite sheet at the reverse side 50b. The exfoliated graphite sheet and the wire net are present in the obverse side 50a in a mixed manner. This sliding sheet material is cut and shaped into a packing or the like in which at least one side or face thereof is completely covered with the exfoliated graphite having insufficient wear resistance, and therefore when the packing is worn into a reduced thickness, this causes a backlash. Besides, when the wire net becomes exposed as a result of wear, there is encountered a problem that the opposite (both) sides of the exfoliated graphite sheet are divided by the meshes of the wire net, so that the exfoliated graphite sheet is susceptible to separation.